CYTOCHROME C 349 



about 18,000 by differential adsorption of ferrocytochrome and ferri- 

 cytochrome on kaolin; by diffusion methods Theorell {2766) found 

 16,500 for cytochrome c with 0.34% iron. Ultracentrifuge and osmo- 

 metric methods yield similar results (8,2131). The molecular weight 

 of pure cytochrome c with 0.43% iron is 13,000, (Pedersen, quoted 

 by Theorell, 2778). 



Absorption spectra. Data on the absorption spectra of cytochrome 

 c are summarized in Table II. 



It is doubtful whether the band at 316 mfj. is a genuine band of cytochrome 

 c. Schales and Behrnts-Jensen (24^35) conclude from it that cytochrome c 

 contains a sulfoxide (SO) group. Dithionite however, wfiich is frequently 

 used in the preparation of cytochrome or for reduction, has a band in this 

 region. This band was not observed in cytochrome c by Lavin and collab- 

 orators {1662). 



TABLE II 

 Absorption Spectra of Cytochrome c 



Absorption maxima" 



Reference 



619,621,l^fi8,2061,2766 

 597,2766 



597,2766 



" On the basis of one gram equivalent iron per liter. 

 '' Theorell (2766) found a higher value (12.0). 



Oxidation-reduction potentials. There is now little doubt that the 

 potential found by Green {lOJ^S) for a cytochrome c prepared from 

 yeast {Eq = 4- 0.125, pH 7.14) was far too low. The earlier value 

 + 0.26 found by Coolidge {J^86) in Conant's laboratory has been con- 

 firmed by several authors {123,1634,2680,3132,3133). Between pH 5 

 and 8 it does not vary with pH. 



Estimation. Three methods are available for the estimation of cytochrome 

 c in tissues. The first measures its catalytic effect on the oxidation of sub- 

 strates such as hydroquinone in the presence of an excess of cytochrome 

 oxidase and of semicarbazide, which combines with the quinone {14Ji.6,1447, 

 2674). In the second, cytochrome c is extracted and freed from accompany- 



